Why is my car getting very shaky while idling? Rpm fluctuates

What is your question?

The car starts up with rpm at around 2 or 3000 then goes dips down to less than 1000 rpm and is very shaky. At some red lights, it shakes as though it will turn off. I'm thinking it could be O2 sensor, catalytic converter, something with the fuel system, or even drive belts. Any professional opinion would ve very helpful!